The present invention relates to an amplifier arrangement comprising an amplifier having an input terminal and a current output terminal for providing an output current in dependence on a signal at the input terminal, the amplifier further comprises a feedback path coupled between the current output terminal and the input terminal.
Such amplifier arrangement is known from the book xe2x80x9cDesign of High-Performance Negative-Feedback amplifiersxe2x80x9d by Erst H. Nordholt, chapter 1, page 8, FIG. 4, Elsevier Publishing Company 1983, ISBN 0-444-42140-8.
Amplifier arrangements according to the preamble are used in several broadband applications, such as CATV systems. The prior art amplifier arrangement is intended for use with a signal source that behaves as a current source. The current forced by the input source into the amplifier, flows through the feedback path and causes a voltage across said feedback path. Due to the feedback arrangement, the current source in the amplifier supplies an output current which matches the sum of the current that flows in the feedback path and the current that flows into the load impedance of the amplifier arrangement.
The prior art amplifier arrangement shows good high frequency behavior. It further has a low output impedance, which even decreases with increasing gain of the amplifier. This low output impedance is not always desired. E.g. in CATV systems it is desired to drive the load impedance with an amplifier having an output impedance that matches the load impedance. In CATV systems this impedance is equal to 75xcexa9. This matching of load impedance and output impedance is desired to suppress reflections at the output of the amplifier of RF signals picked up by the coaxial cables in the CATV system.
An obvious solution to this problem could be connecting a resistor in series with the load impedance in order to obtain a desired value of the output impedance. This however would result in half the power generated by the amplifier being dissipated in said series resistor.
An object of the present invention is to provide an amplifier arrangement according to the preamble having an increased output impedance, without using series resistors dissipating a substantial fraction of the power generated by the amplifier.
To achieve the object of the present invention, the amplifier arrangement is characterized in that the amplifier comprises a further current output terminal for providing a further output current which is dependent on the current flowing through the current output terminal, and in that the second current output terminal is coupled to the output of the feedback network.
By introducing a further current output terminal, and coupling it to the output of the feedback path it is obtained that the voltage drop over the feedback path decreases with an increasing output current of the amplifier. This decrease of the voltage drop over the feedback path has as result that the output voltage of the amplifier arrangement decreases when the output current of the amplifier increases. This effect results in an increased output impedance of the amplifier arrangement. By using an amplifier arrangement according to the present invention is possible to obtain an amplifier arrangement of which the output impedance can be given a predetermined value without needing bulky transformers to obtain an impedance transformation.
An embodiment of the invention is characterized in that the amplifier comprises at least a first and a second transistor, in that a first main electrode of the first transistor being coupled to the current output, in that a first main electrode of the second transistor is coupled to the further current output, in that a second main electrode of the first and the second transistor are coupled to a bias source, and in that a control electrode of the first and second transistor are coupled to the input terminal of the amplifier.
The use of a parallel connected first and second transistor for realizing the current outputs results in a simple amplifier of which the ratio between the two output currents can be chosen by choosing a proper ratio of the emitter areas of the two transistors.
A further embodiment of the invention is characterized in that the amplifier comprises a third transistor, in that a first main electrode of the third transistor is coupled to a supply terminal, in that a second main terminal of the third transistor is coupled to the bias source, in that a control terminal of the third transistor is coupled to a further input of the amplifier, and in that a coupling element is coupled between the further input of the amplifier and a reference terminal.
By adding a third transistor, an amplifier with symmetric inputs is obtained. This makes it possible to use the amplifier arrangement with balanced input signals to convert said balanced signal into an unbalanced signal without needing transformers.
A further embodiment of the invention is characterized in that the feedback path and the coupling element comprise impedance elements with values having a ratio of N/(N+1), in which N is the ratio between the output current and the further output current.
In this embodiment the amplifier can be driven from two balanced current sources.
A still further embodiment of the invention is characterized in that the amplifier further comprises a voltage controlled current source with two input terminals and at least one output terminal, in that the first input terminal of the voltage controlled current sources is coupled to the control electrode of the first and second transistor, in that the second input terminal of the voltage controlled current source is coupled to the control electrode of the third transistor, and in that the output of the voltage controlled current source is also coupled to the control electrode of the third transistor.
In this embodiment the gain of the amplifier is increased due to a positive feedback introduced by the voltage controlled current source. Increasing the gain of the amplifier results in more accurate values of the transimpedance and the output impedance. An advantage of this particular way of increasing the gain of the amplifier is that the high frequency properties are hardly effected. However, care should be taken to ensure stability of the amplifier under all circumstances.